Tissue stimulation using implantable electrodes has important therapeutic uses. For example, implantable pacemakers, defibrillators, and cardioverters stimulate the muscle tissue of the heart. Implantable neurostimulators have been developed to provide therapy for a variety of disorders, as well as other treatments. Implantable neurostimulators can be used in neurological therapy by stimulating nerves or muscles, for example, spinal cord tissue or brain tissue. Other uses of implantable neurostimulators include, but are not limited to, treatment for urinary or faecal urge incontinence by stimulating nerve fibers proximal to the pudendal nerves of the pelvic floor, treatment for erectile and other sexual dysfunctions by stimulating the cavernous nerve(s), treatment for reduction of pressure sores or venous stasis, etc. Other uses for implantable electrodes include muscle stimulation, gastroparesis treatment, wound healing, retinal and sub-retinal treatment, recording, sensing, and monitoring.
Stimulation systems typically include implantable electrodes attached to, or disposed adjacent to, the tissue to be stimulated. Some stimulation systems, including at least some spinal cord stimulation systems, have an implantable percutaneous, cuff, or paddle lead with multiple electrodes, as well as a separate control module that houses the power source and pulse generator. In many configurations, this control module is also implantable.
Implantable microstimulators, such as the BION® device (available from Advanced Bionics Corporation, Sylmar, Calif.), have exposed electrodes and a small, often cylindrical, housing that contains the electronic circuitry and power source that produce electrical pulses at the electrodes for stimulation of the neighboring tissue. Once implanted, it is often preferable that the microstimulator can be controlled and/or that the electrical source can be charged without removing the microstimulator from the implanted environment.
In many instances, implantable electrodes for electrical stimulation ideally should have a relatively small geometric surface area to produce a lower stimulation threshold and longer battery life. However, the reduction of the geometric surface area can increase current density and possibly exceed safe charge injection limits, which could result in, for example, dissolution of electrode material, undesirable electrolytic redox reductions, and production of toxic chemicals. This can be counteracted by increasing the actual surface area by, for example, using porous electrode materials such as platinized platinum, iridium oxide, titanium nitride, or sintered microspheres, or by using electrodes with fractal surface morphology or fractal coatings.